ABSTRACT
Cartilage tissue is a peculiar type of connective tissue; it plays an important role in the organization and formation of the vertebrate skeleton, as well as of some extraskeletal structures. Three different types of cartilage tissue – hyaline, elastic and fibrous – are present in the organism, forming the hyaline, elastic and fibrous types of cartilage, respectively. Despite their significant morphological and biochemical differences, these have in common the predominant presence of two main macromolecular components in the extracellular matrix – the large type II fibrillar collagen and the large proteoglycan, aggrecan. These two components, expressed by cells of the specific chondroblastic differon, impart unique biomechanical properties to cartilage tissue that define its biological significance. The morphological, biochemical and molecular-biological characteristics of cartilage tissue are most clearly represented in hyaline cartilages, also called “true”. The biochemical and molecular-biological characterization
of cartilage tissue in this chapter is therefore based primarily on studies of hyaline cartilages, more specifically, articular cartilage, which have been studied in great detail. It should be emphasized that, despite some differences among the different types of cartilage, the outlined concepts, to a significant degree, apply to the elastic and, to some degree, to the fibrous cartilages as well. The presence of elastic fibers in the extracellular matrix distinguishes elastic from hyaline cartilages, and imparts elasticity upon the structures comprised of such cartilages. Fibrous cartilages are complex fibrous structures built mostly of dense fibrous connective tissue, which contain regions or zones of hyaline cartilage tissue. These regions contribute to the biomechanical properties of fibrous cartilages. The formation of virtually all bones of the axial skeleton (except for parts of the clavicle), and of all limb bones, proceeds through the stage of cartilage models. The models are formed from mesenchymal condensation cells undergoing chondroblastic differentiation (see section 10.2.2). The models’ cartilage tissue – called provisory (from the Lat. provideo – to foresee) or transitory, i.e. preliminary, temporary – is substituted by bone tissue at
Table 7.1 Quantitative biochemical parameters of human femoral head articular cartilage
birth, and persists only in the so-called growth plates (metaepiphyseal cartilage of long bones) (see 10.3) during the postnatal period until the onset of puberty and until the organism completes growth. The transitory cartilage tissue resembles hyaline cartilage in terms of its structural and biochemical characteristics. All types of permanent cartilage tissue – hyaline, fibrous and elastic (Fig. 7.1) – possess common morphological, chemical and biochemical characteristics, with some morphological and chemical distinctions (Wachmuth et al., 2006). First, the common characteristics pertain to all types of connective tissue (see 1.3). The predominance of the amount of extracellular matrix over the amount of cellular components is the most important common characteristic of all permanent types of cartilage tissue. This prevalence is so significant that quantitative biochemical data of tissue samples, with the exception of nucleic acid concentrations, actually reflect the composition of the matrix. This pattern is illustrated in Table 7.1, which presents the results of a quantitative biochemical analysis of the most thoroughly studied type of hyaline cartilage –the articular cartilage (Pavlova et al., 1988).
